Phonotactic patterns are commonly constrained by morphology. In English, for example, non-homorganic nasal–stop sequences are disallowed within morphemes but may occur across morpheme boundaries. This article demonstrates that similar effects of morphology on phonotactics can be found with non-concatenative morphology, even though they involve morphological domains that are more difficult to identify on the surface. Specifically, vowel alternation in a class of Egyptian Arabic verbs is affected by gradient phonotactic restrictions on consonant–vowel co-occurrence. However, such restrictions are only active in the imperfective form (e.g., [-rgaʕ] ‘return.ipfv’), not the perfective (e.g., [rigiʕ] ‘return.pfv’). Using a lexicon study and a wug test, I show that this pattern is in fact bounded by morphological domains and is reliably generalised by speakers when deriving novel forms. I compare accounts of this effect that differ on whether they require abstract morphosyntactic representations and non-concatenative morphemes and discuss their implications.

Tone is distinct from other phonological phenomena both qualitatively and quantitatively (Hyman 2011), and has been instrumental in shaping phonological theory in many ways. However, the contributions to current linguistic theory of “grammatical tone’ (GT) – a type of non-concatenative morphology where a morpheme is expressed at least in part by tone and/or tone changes – have been less apparent. In this paper, we take stock of the types of GT patterns attested in the literature and the different theoretical treatments of GT that have been proposed to date. We show that GT is still to a large extent underexplored, and highlight the immense potential of the study of GT for improving our understanding of phonology and its outer limits. This paper serves as an introduction to the high-quality research articles collected in this special issue, which directly address how GT critically informs phonological theory and its current developments.